The present invention relates to the analysis of fluids. The present invention relates more particularly to the qualitative analysis of fluids, and particularly of hydrocarbon- or biofuel-based fuels, ensuring the operation of a combustion engine such as those on motor vehicles.
Motor vehicles are more and more equipped with an electronic management system configured to optimize the operation of the engine according to different parameters measured by sensors. Such systems are generally configured to adjust the quantity of fuel injected into the engine, set the injection time, the ignition advance, the intake pressure and the recycling of exhaust gas, according, in particular, to the engine speed, the temperature of oil and coolant, and external parameters such as the atmospheric pressure and ambient temperature.
However, such optimization is limited by fluctuations in the quality of fuels. Indeed, although they are defined by standards, fuels are subject to significant variations in quality depending on the weather and the filling station. Fuels are generally characterized by physico-chemical parameters such as octane ratings and the vapor pressure for spark ignition engines, the cetane rating, the flashpoint and resistance to low temperatures for diesel engines, as well as the distillation curve, the density and the oxygenated compound content. It is thus estimated that the physico-chemical parameters of hydrocarbon-based fuels can vary by 15 to 40% or more, around standardized average values specified in standards.
Now, the operation of a combustion engine is optimized for standardized fuels. If the quality of the fuel taken at the pump is too different from the standardized fuel quality, such optimization is no longer done, and the engine will tend to consume more fuel and generate more polluting gas.
Therefore, it is desirable to determine the quality of fuel supplying the engine and to take the results obtained into account to manage the operation of the engine. For that purpose, near-infrared absorption spectrometry is suited to the qualitative assessment of a hydrocarbon or a hydrocarbon mix.
A standard spectrometer generating a fluid absorption spectrum generally comprises the following elements:
a light source covering at least one wavelength band in which the measurements must be taken,
a device referred to as a “probe” in the following description, in which the light produced by the light source and the fluid to be analyzed interact, and
a spectrum analyzing sensor which analyses the light at the output of the probe.
Such a spectrometer enables an absorption spectrum to be obtained in the form of a curve of T=f(λ) type, indicating the quantity T of light having passed through the fluid to be analyzed according to the wavelength λ.
A spectrometer is characterized mainly by its spectrum analysis range (width and position of the spectra generated), its precision of analysis or the number of measurement points constituting the spectra supplied, its accuracy of measurement for the absorption value and its sensitivity, i.e. its capacity to measure low quantities of light.
Present-day spectrometers are generally very complex and thus very expensive, and are relatively large in size. In particular, present-day spectrometers comprise many optical components (lenses, filters, prisms, minors, diffraction gratings) separated by air spaces. The alignment of these components is critical for the spectrometer to function correctly. Therefore, present-day spectrometers, generally designed for laboratories, are not suited to the environment of a combustion engine or a motor vehicle. In particular, they are not designed to withstand the intense vibrations generated by the combustion engine or the motor vehicle. Indeed, between the light source and the spectrum analyzing device, the light beam passes through a significant number of optical components separated by air spaces. Each interface between an optical component and the air is a potential source of misadjustment or deterioration.
The light sources traditionally used in spectrometry (incandescent or metal vapor lamps, halogens, etc.) are not compatible with the requirements of robustness, service life and size required in an application on board a combustion engine or in a vehicle. The use of lasers must also be ruled out because the same number of lasers as measurement points in the desired spectrum would be necessary. The use of lasers must also be ruled out for reasons of service life, stability and robustness in a hostile environment.
Furthermore, French patent application publications No. FR 2 798 582 and FR 2 789 294 considered using light-emitting diodes associated with standard optical components.
U.S. Patent Application Publication No. 2007/0084990 describes a miniaturized spectrometer, integrating a source of energy and detectors in a single box which contains optical components for interfacing with the sample, and acquisition and processing circuits. The spectral selectivity is obtained by continuously variable optical filters or by a matrix filter. This device enables transmittance, absorption, turbidity and fluorescence to be measured. The document provides for using narrow- or broad-spectrum light sources such as white LEDs.
French patent application publication FR 2 583 164 describes a device comprising a light source associated with focusing optics to illuminate a fluid through an optical fiber. The light is collected from the fluid using another optical fiber which sends the light to a color separation device.
British published patent application GB 2 283 091 describes a liquid spectrum analyzing device for analyzing the spectrum of liquids comprising an optical fiber which sends a ray to a probe comprising a reflector and a passage for the liquid to be analyzed. The reflector reflects the ray in the fiber to an analyzing unit.
U.S. Patent Application Publication No. 2005/0140270 describes a light-emitting device comprising several sources of red, green or blue lights, such as LEDs. The light emitted by each source is concentrated by an element of an array of Concentrators, then sent by a respective light guide of a bundle of light guides.
U.S. Pat. No. 6,560,038 describes an optical coupler associated with an LED or a set of LEDs.
French patent application publication FR 2 566 909 describes a device for detecting a product in a liquid comprising a bundle of optical fibers a part of which sends light to the liquid, a mirror sunk in the liquid which reflects the light to the bundle of fibers, the other part of the bundle of fibers being linked to a detector.
U.S. Pat. No. 4,989,942 describes a device for analyzing a liquid comprising a light source sent by an optical fiber to a collimating lens. The light then passes through the liquid before being collected by another collimating lens. The collected light is then sent by an optical fiber to an analyzing device.